WEBVTT 00:00:00.500 --> 00:00:03.976 Welcome to GCSE Edexcel Science revision. 00:00:04.126 --> 00:00:07.639 Unit C P 11: Electromagnetic induction. 00:00:10.300 --> 00:00:18.224 A changing magnetic field through a coil or relative motion that cuts magnetic field lines can induce a potential difference. 00:00:18.374 --> 00:00:20.785 This is electromagnetic induction. 00:00:23.467 --> 00:00:27.745 Moving a magnet into or out of a coil changes the field through the coil. 00:00:27.895 --> 00:00:32.982 A sensitive voltmeter connected across the coil can detect an induced voltage. 00:00:33.132 --> 00:00:37.813 The voltmeter measures potential difference, with polarity set by connections. 00:00:42.467 --> 00:00:47.842 If the coil is part of a closed conducting circuit, the induced voltage drives a current. 00:00:47.992 --> 00:00:52.143 An open circuit can have induced voltage but no continuing current. 00:00:54.800 --> 00:01:03.343 A stationary magnet held within a stationary coil does not produce a continuing induced voltage because the field through the coil is not changing. 00:01:06.000 --> 00:01:13.294 Faster magnet movement, a stronger magnet or more coil turns can increase induced voltage when the other factors are fixed. 00:01:15.967 --> 00:01:21.417 Reverse the motion or reverse the magnet's poles to reverse the induced voltage direction. 00:01:21.567 --> 00:01:24.722 Reversing both can keep the direction unchanged. 00:01:27.400 --> 00:01:32.125 The field associated with the induced current opposes the original change. 00:01:32.275 --> 00:01:41.799 An approaching north pole induces a facing north pole that resists approach; a withdrawing north pole induces a facing south pole that resists separation. 00:01:41.949 --> 00:01:45.731 Both induced interactions resist the change that created them. 00:01:50.400 --> 00:01:56.895 This opposition means mechanical work is needed to maintain motion when electrical energy is transferred. 00:01:57.045 --> 00:01:59.498 Induction does not produce free energy. 00:02:02.167 --> 00:02:06.606 Connect a coil to a sensitive centre-zero voltmeter or data logger. 00:02:06.756 --> 00:02:12.387 Move a bar magnet in and out and record size and sign of voltage; compare with holding it still. 00:02:15.067 --> 00:02:19.887 Change one factor at a time: magnet speed, magnet strength or number of turns. 00:02:20.037 --> 00:02:25.157 Keep coil size, movement path, starting positions and other variables consistent. 00:02:27.833 --> 00:02:34.093 For fair comparisons, use repeatable movement and compare the same part of the movement or peak voltage. 00:02:34.243 --> 00:02:37.826 Repeat trials because hand movement gives varying speed. 00:02:40.500 --> 00:02:45.437 Investigate direction by reversing magnet motion or poles individually. 00:02:45.587 --> 00:02:49.723 Label which end of the coil is connected to the positive voltmeter terminal. 00:02:52.400 --> 00:02:59.118 A voltage pulse depends on the rate of field change, not simply how much magnetic material is sitting inside a coil. 00:03:01.800 --> 00:03:06.803 A transformer has a primary coil and a secondary coil linked by a magnetic core. 00:03:06.953 --> 00:03:10.628 The coils are electrically separate in the usual ideal model. 00:03:10.778 --> 00:03:14.305 A changing magnetic field transfers energy between them. 00:03:14.455 --> 00:03:18.573 Coils are magnetically coupled, not joined by an electrical wire. 00:03:23.233 --> 00:03:28.427 An alternating current in the primary creates a changing magnetic field in the core. 00:03:28.577 --> 00:03:33.109 The changing field through the secondary induces an alternating voltage. 00:03:35.767 --> 00:03:39.210 A secondary current flows if a load completes its circuit. 00:03:39.360 --> 00:03:45.215 An unloaded secondary can have voltage with little output power because there is little load current. 00:03:47.867 --> 00:03:54.447 A steady D C input does not provide the continuously changing field needed for normal transformer operation. 00:03:54.597 --> 00:04:00.399 Switching D C on or off can produce brief changes, not a sustained transformed output. 00:04:03.067 --> 00:04:08.421 A step-up transformer increases A C voltage; a step-down transformer decreases it. 00:04:08.571 --> 00:04:14.031 More secondary than primary turns gives step-up behaviour; fewer gives step-down behaviour. 00:04:16.700 --> 00:04:19.747 The core helps the magnetic field link both coils. 00:04:19.897 --> 00:04:28.065 Real transformers transfer some energy to the surroundings, including by heating, so their useful output power is less than their input power. 00:04:30.733 --> 00:04:36.056 For a transformer assumed one hundred percent efficient, input power equals output power. 00:04:36.206 --> 00:04:45.164 Use primary voltage times primary current equals secondary voltage times secondary current, with voltages in volts and currents in amperes. 00:04:47.833 --> 00:04:53.399 A primary of 230 volts and 2 amperes supplies 460 watts. 00:04:53.549 --> 00:05:00.993 If the ideal secondary is 23 volts, its current is 460 divided by 23 equals 20 amperes. 00:05:01.143 --> 00:05:04.883 Lower output voltage permits greater current at the same power. 00:05:09.533 --> 00:05:15.697 Increasing voltage reduces current for the same transferred power; reducing voltage increases current. 00:05:15.847 --> 00:05:21.957 A transformer cannot increase both output voltage and output current at unchanged input power. 00:05:24.633 --> 00:05:29.029 Rearrange the ideal equation to find a missing voltage or current. 00:05:29.179 --> 00:05:36.237 For example, secondary current equals primary voltage times primary current divided by secondary voltage. 00:05:36.387 --> 00:05:40.486 Check whether the question explicitly assumes an ideal transformer. 00:05:43.167 --> 00:05:50.003 If a real transformer has known efficiency, useful output power equals efficiency fraction times input power. 00:05:50.153 --> 00:05:53.701 Account for losses rather than claiming exact equality. 00:05:56.367 --> 00:06:03.821 The National Grid links generation and electricity transmission; local distribution networks deliver energy to consumers. 00:06:03.971 --> 00:06:07.775 Generation includes power stations and renewable sources. 00:06:10.433 --> 00:06:14.631 Step-up transformers raise voltage before long-distance transmission. 00:06:14.781 --> 00:06:18.052 For the same power, higher voltage means smaller current. 00:06:20.733 --> 00:06:24.473 Cable heating loss is power loss equals I squared times R. 00:06:24.623 --> 00:06:31.331 Reducing current greatly reduces heating losses for the same cable resistance, improving transmission efficiency. 00:06:31.481 --> 00:06:35.157 Halving current reduces I squared times R loss to a quarter. 00:06:39.833 --> 00:06:45.197 Step-down transformers reduce voltage for distribution and again near domestic users. 00:06:45.347 --> 00:06:54.359 UK domestic supply is approximately 230 volts A C; high transmission voltages would be unsuitable for direct household use. 00:06:54.509 --> 00:06:59.995 Illustrates the purpose of voltage changes rather than a universal list of grid voltages. 00:07:04.667 --> 00:07:10.277 Some transmission lines operate at hundreds of kilovolts, for example 400 kilovolts. 00:07:10.427 --> 00:07:17.284 Intermediate and generator voltages vary; the source's numbers are examples rather than one compulsory sequence. 00:07:19.967 --> 00:07:24.958 High-voltage equipment requires insulation, separation and controlled access. 00:07:25.108 --> 00:07:29.325 Lower domestic voltage still presents a serious electrical hazard. 00:07:32.000 --> 00:07:35.374 The grid transfers energy; it does not create it. 00:07:35.524 --> 00:07:41.730 Demand, generation and network operation must be managed, with some energy inevitably dissipated. 00:07:44.400 --> 00:07:47.055 That completes Electromagnetic induction. 00:07:47.205 --> 00:07:50.551 Revisit the notes and test yourself on the revision website.